The formation of various integrated circuit (IC) structures on a wafer often relies on lithographic processes, sometimes referred to as lithography or photolithography. For instance, patterns can be formed from a photo resist (PR) layer by passing light energy through a mask (or reticle) having an arrangement to image the desired pattern onto the PR layer. As a result, the pattern is transferred to the PR layer. In areas where the PR is sufficiently exposed and after a development cycle, the PR material can become soluble such that it can be removed to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, etc.). Portions of the PR layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer. The exposed portions of the underlying layer can then be etched (e.g., by using a chemical wet etch or a dry reactive ion etch (RIE)) such that the pattern formed from the PR layer is transferred to the underlying layer. Alternatively, the PR layer can be used to block dopant implantation into the protected portions of the underlying layer or to retard reaction of the protected portions of the underlying layer. Thereafter, the remaining portions of the PR layer can be stripped.
There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. As a result, there is a corresponding need to increase the resolution capability of lithography systems. One promising alternative to conventional optical lithography is a next-generation lithographic technique known as immersion lithography. In immersion lithography, a liquid immersion medium is placed between the final optical element of a lithography system and the wafer to be imaged. The patterned light is transmitted to the wafer through the immersion medium. The immersion medium replaces a gaseous or vacuum chamber gap that is conventionally present between the final lens of a dry lithography imaging system and the wafer. Immersion lithography has been found to enhance imaging of the wafer by increasing the refractive index of the material disposed between the final element of the projection system and the wafer. Also, the effective numerical aperture of the system can be increased, which can lead to an increase in depth of focus.
However, attempts to implement immersion lithography have encountered a number of challenges. For example, contaminants in the immersion medium and/or on parts that control the flow of the immersion medium can adversely affect the quality of the exposure pattern incident on the wafer. One particular problem has been the growth of biological contaminants (e.g., bacteria, algae, etc.) on and in parts that come in contact with the immersion medium. In the past, attempts have been made to control the presence of biological contaminants by using hydrogen peroxide or by using an ozone cleaning method. However, these techniques involve the introduction of chemicals into at least a part of the lithography system, which can lead to increased wear of system components and/or can lead to the introduction of defects when imaging a wafer.
Accordingly, there exists a need in the art for improved immersion lithography systems and associated methods of controlling biological contamination in the immersion lithography systems.